1. Field of The Invention
The present invention relates to an integrated circuit incorporating a constant-voltage control circuit and more particularly to an integrated circuit which incorporates part of a constant-voltage circuit and is suitable for use in such an environment that supply voltage exceeds withstand voltage in the elements of the integrated circuit.
2. Description of Conventional Art
Along with attempts to make products small in size, there has developed a persistent demand for their circuit miniaturization. In order to meet such a demand, efforts are also being made to microminiaturize integrated circuits recently. As a result, circuits such as data processing circuits and signal processing circuits including those which are considerably complicated in construction have been integrated into one-chip integrated circuits. The signal processing circuit on the periphery of the recording head of a video tape recorder, for instance, is seen to demonstrate the feasibility of integration into an integrated circuit as far as that circuit portion is concerned.
In this case, most of the portion requiring space in the circuit is occupied by a drive circuit for driving mechanical parts such as a head and the most supply power is also consumed by the drive circuit.
Therefore, voltages fit for use in the drive circuit, for instance, 12 V, 24 V and so on, are necessarily used as those of the supply power in such an environmental condition on one hand, and voltages to be supplied to an integrated circuit have to be within a range of voltages lower than the maximum rated voltage in the process employed because the withstand voltage of the signal processing IC is reduced as the integration of the internal microminiaturized elements increases.
Two kinds of power supplies are thus necessitated since the supply voltage intended for the signal processing IC differs from what is intended for the drive circuit. However, it is still uneconomical to provide an individual power supply for each integrated circuit designed to consume trivial power. For this reason, the practice often followed to cope with the inconvenience is to employ a constant-voltage circuit for locally generating the power intended for the signal processing IC from the power supply intended for the drive circuit, whereby a single power supply is made useful without causing problems by supplying the power thus locally generated to the signal processing IC.
FIG. 2 is a block diagram of a conventional constant-voltage circuit for use in the case stated above.
In FIG. 2, numeral 1 denotes a current control circuit; 2, a voltage detection circuit; 3, a reference voltage generating circuit; 4, a differential circuit; 5, an amplifier circuit; and 6, an error amplifying circuit.
The current control circuit 1 receives power at unregulated voltage to be supplied to the aforementioned drive circuit or power at what is equivalent thereto as an input voltage U and generates power at an output voltage V by controlling the resulting current by means of a control element such as a transistor, and supplying the power at the voltage thus reduced to a smoothing capacitor. On receiving a feedback control signal D at this time, the current control circuit 1 increases and decreases the quantity of the current in response to the control signal D in order to control the output voltage V.
On receiving the output voltage V, the voltage detection circuit 2 generates a detection signal A having a waveform which accords with the output voltage V by means of resistance division.
The reference voltage generating circuit 3, which comprises a Zener diode, for instance, generates a reference voltage signal R for determining a target control value of the output voltage V.
On receiving the detection signal A and the reference voltage signal R, the differential circuit 4 in the error amplifier circuit 6, which comprises the differential circuit 4 and the amplifier circuit 5, discriminates between the detection signal A and the reference voltage signal R, and generates an error signal B. On receiving the error signal B, the amplifier circuit 5 amplifies the signal while converting it to a current signal suitable for driving the transistor of the current control circuit 1, and feeds back the amplified signal to the current control circuit 1 as the control signal D.
The power at the output voltage V thus reduced from the input voltage U is controlled by the feedback system operating in such a way that any change of the voltage value from the target value predetermined by the reference voltage signal R is offset. Consequently, the output voltage is held constant at all times.
Provided the constant voltage circuit together with the signal processing IC which requires such a low voltage power supply is locally installed, it is unnecessary to provide two power supply systems which is uneconomic.
Presently, so-called three-terminal regulators with a conventional constant-voltage circuit integrated into one chip have been put on the market and they are available for use at low prices.
However, these products sold on the market are intended for general purpose and not applicable to precision signal processing circuits for video tape recorders and the like because they are hardly capable of removing input noise and ripple components.
Nevertheless, the constant-voltage circuit constructed of discrete parts becomes too complicated to be packaged and still poses a problem as a demand for miniaturizing the apparatus cannot be met.